I've heard people make this argument before but riven's CC isn't all that great, mostly due to the fact you build no tank items, and you also have the shortest stun in the game and i almost feel like her knock up is shorter than others too with both her stun and knock up with small radiuses, And in reality if you really wanted a lot of hard CC u would pick maokai, nautilus, thresh, hecarim, those guys who can build really tanky, constantly put out CC, still do damage without building a whole lot of it, and alot more forgiving when they fall behind also because their kits allow them to be useful.
So in reality riven does only bring damage, she's also shit at siegeing because she has no wave clear. Also, her team fight isn't that great because of her being an ad caster stun or roots make her completely useless. Last thing is riven can only snowball, if she falls behind she's useless, and there are a lot worse people to have fed than riven.